Voltage Sags, Single phase and multi-phase sags

Voltage sags are usually because of the faults in the power system. Sort circuit interruptions are typically associated with switch gear operation related to the occurrence and termination of short circuits in the system. They are characterized by the resulting voltage variations outside the normal operating range of voltages.  A power voltage sag is a short-duration (typically 0.5 to 30 cycles) reduction in rms voltage caused by faults on the power system and by the starting of large loads. Momentary interruptions (typically not more than 2 to 5 seconds) cause a complete loss of voltage and are common because of the remedial actions taken by utilities to clear transient faults on their systems. Sustained interruptions of longer than one minute are normally due to permanent faults.In this article we will know about the voltage sags also learn about single phase and multi-phase sags . From this article we will learn in detail abut overall sags.

Voltage Sags. What is Voltage Sags ?

Voltage sags and momentary interruptions are two of the most important power quality concerns for consumers. They understand that interruptions cannot be completely prevented. However, they are less tolerant when their equipment misoperates due to momentary disturbances which can be more frequent than complete outages. The impact of the consumer depends on the voltage magnitude during the disturbance, the duration of the disturbance and the sensitivity of the equipment.  Voltage variations and interruptions are inevitable on the power system. Since it is impossible to eliminate the occurrence of faults, there will always be voltage variations.

Complaints about the quality of power due to sags and  interruptions are increasing. There are a number and industrial) have more sensitive loads. Industries are relying more on automated equipment to achieve maximum productivity. Whether or not a voltage sag causes a problem will depend on the magnitude and duration of the sag and on the sensitivity of the equipment .Important equipment sensitive to voltage sage includes VSD, programmable logic controllers, controller power supplies and control relays. Computerized controls tend to lose their memory and the processes that are being controlled also become complex. Thus, an interruption can lead to very expensive downtime.

As said earlier, a power system faults is a typical cause of a voltage sag. Fault occur in transmission, distribution systems and the sag propagate throughout the power system. Because of the meshed structure of transmission system, the sagged area caused by transmission faults is normally large. The sag distribution caused by transmission fault is highly dependent on location of the supply point in the transmission system. In addition, if local generation exists this will also have a strong effect on the sag distribution.

In principle, faults on all voltage levels contribute to the sag distribution experienced by a low voltage consumer but, because of network characteristics, the sags caused by fault in some part of power system are shallow or negligible. Most of the fault affecting low voltage consumers occur in medium voltage network. A low voltage consumers experiences sags caused by faults in the neighboring medium voltage feeders and also via the high voltage systems from faults in the MV networks located behind the neighboring substation. The duration of voltage sags is mainly determined by the operating time of the device which disconnects the fault from the system. These devices are mainly fuses, circuit breakers and protection relay and fuses have an inverse time characteristic.

Single-phase and Multi-phase Sag

The most common voltage sags (70%)  are single-phase events which are typically due to a phase to ground fault occurring somewhere on the system. This phase to ground fault appears as a single-phase voltage sag on the feeder from the same substation. It is common to see single-phase voltage sags to 30% of nominal voltage or even lower.

Symmetrical three-phase sags account for less than 20% of all sag events and are caused either by switching or tripping of a 3-phase circuit breaker or  reclose which will create a 3-phase voltage sag on other lines fed from the same sub-station . 3-phase sags are also caused by starting large motors but this types of event typically causes voltages sags to approximately 80% of nominal voltage and are usually confined to an industrial plant or its immediate neighbors.

Voltage-Sag Characteristics

The characterstic of sag depends. The parameters used to characterize voltage sag are magnitude, duration unbalance and phase angle jump.

Magnitude of Sag-

One common practice is to characterize the sag magnitude trough the remaining voltage during the sag, known as “retained or residual voltage”. Magnitude of a voltage sag is the value of residual voltage during the event. The magnitude of voltage sag can be expressed in a number of ways like one cycle or half cycle rms voltage, magnitude of fundamental component of voltage sag and peak voltage over each cycle or half cycle.

Duration of Sag :  Duration of voltage sag is the time for which the rms value of voltage stays below a voltage dip threshold. The duration of voltage sag is mainly determined by the fault clearing time. Normally system, which affects the duration of faults depending on its location in the system.

Unbalance of Sag : Voltage sag due to faults in the system can be symmetrical or unsymmetrical depending on the type of fault. Due to three-phase fault the sag will be symmetrical but due to single phase, double phase or double-phase to ground faults the sag in three phases will not be symmetrical and are called as unbalance sag.

Phase-angle Jump

Phase-angle jump manifest itself as a shift in zero crossing of the instantaneous voltage during fault. Phase-angle jump during 3-phase faults are due to the difference in X/R radio between the source and the feeder.

Effects of Voltage Sag on Motors and Variable Speed Drives.

Most of the industrial motors are induction motors. Synchronous motors are used exclusively in constant speed drives. Induction motors are protected against the effects of short-duration voltage changes because of the inertia of their own and that of the driven machine. Because of  a voltage sag, the electromagnetic torque of on an electromagnetic of equilibrium between the motor torque and load torque is attained.

Estimating Voltage Sag Performance

Understanding the voltage sag performance of the system and the equipment sensitivity to these events is essential. It is important to understand the expected voltage sag performance of the supply system so that facilities can be designed and equipment specifications can be developed to ensure the optimum operation. Voltage sag analysis is the process of determining the number severity of voltage sags that affects end-use equipment.

Equipment sensitivity to Voltage sags

It is important for a consumer to understand the sensitivity of their equipment to momentary interruptions and voltage sag. Equipment sensitivity to voltage sags depends on the specific load type, control settings and applications.

Hence these are Voltage sags , single phase and multi-phase sags. If you will find any incorrect in above article you must comment below in comment box.